This invention relates to spacecraft, and more particularly to spacecraft in which thrusters consume propellant or fuel from pairs of fuel tanks.
Spacecraft and artificial satellites are in widespread use for communication purposes and for remote sensing. The cost of manufacture and launch of a spacecraft is high. Consequently, once a spacecraft is in orbit, it is important to maintain it in operation for as long as possible. Many spacecraft are required to maintain certain attitudes and/or stations in order to properly perform their function. Electrically operated devices such as reaction wheels or magnetic torquers may be used to control attitude, and require no fuel or other expendables, but instead extract the required energy from solar panels. Even when reaction wheels or magnetic torquers are used, however, chemical monopropellant or bipropellant thrusters may be provided to allow unloading of momentum from the reaction wheels, to provide higher torque than that available from a magnetic torquer, or both. Additionally, reaction wheels and magnetic torquers are not suitable for stationkeeping maneuvers, so chemical thrusters may also be provided to maintain station.
Chemical thrusters include monopropellant and bipropellant thrusters, and also includes arcjets, in which electrical energy is supplied to the chemical reaction to increase the jet velocity. Each of these requires the use of propellant, which is loaded onto the spacecraft at the time of launch, and which is used sparingly in order to maximize lifetime. The end of the useful life (EOL) of a spacecraft is ordinarily taken to be that time at which the propellant available for attitude control or for stationkeeping is exhausted. It should be noted that a small amount of propellant may be held in reserve for use in ejecting the spacecraft from its orbital position at the end of its useful life.
In order to enhance the operational reliability of the spacecraft, many of the operating systems are provided with redundancy. For example, it is customary to provide more chemical thrusters than the minimum necessary to achieve the desired control, and to arrange the thrusters so that a single failure does not make attitude control and stationkeeping impossible. A common scheme is to provide thrusters in "odd" and "even" mutually redundant sets or half-systems. Similarly, the propellant (fuel in the case of a monopropellant engine or fuel and oxidizer in the case of a bipropellant engine) tanks are provided in pairs, and their interconnections with the thrusters are also provided in pairs.
A prior art spacecraft using monopropellant thrusters might include a pair of fuel tanks containing liquid fuel, and pressurized by pressurant gas flowing through a manifold or plenum to the two tanks. Helium is often used as a pressurant gas. The fuel output ports of the two tanks are coupled together by the two branches of a manifold, which connect at a common junction to a thruster supply manifold. The pressurant tank also pressurizes third and fourth fuel tanks, which supply fuel through the two branches of a second manifold to the supply manifold of a further set of monopropellant thrusters. As so far described, the only common element of this prior art arrangement is the pressurant tank. If desired, dual pressurant tanks may be provided, but the potential for failure of the pressurant tank is deemed to be low. Each of the two thruster sets of the above-described prior art arrangement may be operated independently to provide attitude control and stationkeeping. In the event of certain types of failure of a thruster of one of the thruster sets, such as a thruster control valve which cannot be closed or which leaks, a valve is closed in the manifold feeding that thruster, and that thruster set is no longer used. This leaves unused fuel in the tanks associated with the unusable thruster set. Valves in crossover pipes extending between the two half-systems may be opened under these conditions, so the remaining operable thruster set may use the fuel from both sets of tanks.
While many redundancy schemes are possible, a very common characteristic of such systems is the use of a pair of propellant tanks interconnected by a manifold, by which propellant is supplied to the thruster. In order to make use of the largest amount of the propellant stored in the tanks, it is desirable to draw equal amounts of fuel from each tank during each operation of the thrusters. The use of a common source of pressurant gas for each pair of tanks aids in maintaining equal pressure and therefore equal fuel flow. Also, the temperature difference between tanks of a pair is controlled and minimized by a differential heater controller or its equivalent. Efforts are made to match fluid flow conductances through the manifolds, plumbing and components such as filters, check valves and other valves. However, temperature differences still exist between propellant tanks of a pair, and fluid flow conductances are never perfectly matched. As a result, one of the tanks may be depleted of fuel (or propellant in the case of a bipropellent thruster) before the other.
When one propellant tank of a pair is depleted of liquid propellant before the other, pressurant gas appears at the junction point of the manifold. This tends to introduce one or more bubbles of gas into the liquid propellant flowing to the thrusters. Since the pressurant gas is ordinarily less dense and less viscous than the liquid fuel or propellant, the gas tends to flow to the thrusters in preference to liquid. The presence of a gas bubble may be disadvantageous, because the flow of gas through an arcjet can extinguish the jet, and other thrusters become inoperative during expulsion of the gas. However, the situation may be more serious than temporary inoperability during expulsion of a gas bubble, because once pressurant gas starts to flow, the gas continues to flow and may become depleted, leaving no pressurant remaining. When the pressurant is depleted, no pressure remains to drive the propellant from the tank which still contains liquid propellant. Thus, the depletion of propellant in one of a pair of interconnected propellant tanks can signify the end of the propellant supply from the tank pair, even though residual propellant remains in one tank. Thus, depletion of propellant in one tank of a pair may represent the practical end of life of the spacecraft.
The amount of unusable residual propellant in a modern large communication satellite is estimated to be in the neighborhood of 25 lbm, which may be sufficient for operation for several months. The value of an operating satellite may be on the order of one million dollars per week. An improved arrangement for the use of residual fuel is desirable.